President Donald Trump listens to translation during a joint press conference with Japanese Prime Minister Shinzo Abe.Mario Tama/Getty ImagesBuilding a "big, beautiful wall" on the US border with Mexico was a cornerstone of President Donald Trump's campaign.

When supporters cheered on the prospect at Trump's raucous rallies, he would ask who was going to pay for it, and the crowd would shout, "Mexico!"

Now that he's in office, Trump has conceded that Mexico may not pay for the wall, at least at first, instead footing American taxpayers with the bill. Mexico President Enrique Peña Nieto has said his country will not pay for it.

On Saturday, Trump acknowledged reading reports of ballooning cost estimates for the border wall. A US Department of Homeland Security internal report this week pegged the potential cost at around $21.6 billion. But Trump qualified those estimates by saying he hasn't gotten involved in the negotiations to date.

"I am reading that the great border WALL will cost more than the government originally thought, but I have not gotten involved in the design or negotiations yet," the president tweeted on Saturday. "When I do, just like with the F-35 FighterJet or the Air Force One Program, price will come WAY DOWN!"